1. Field of the Invention
The present invention relates to a method for motion-compensated prediction of moving images or pictures using an interpolation method. It also relates to a device for performing the said method.
2. Prior Art
Standard methods for coding of moving images or pictures (H. 263, MPEG-2, MPEG-4, etc) are based on the principle of the so-called hybridized coding, as described in ISO/IEC 14496-2, “Final Draft International Standard of MPEG-4”, Atlantic City, October 1998, MPEG98/N2502. FIG. 1 is a block diagram of a hybrid video-encoder with motion-compensated prediction. The actual picture signal s(t) to be coded with the help of the motion-compensated prediction (motion compensation MC) is predicted from the previously transmitted reference picture s′(t−1). The motion-compensated prediction is performed with the help of a so-called block-wise motion vector d(t), which is determined with the help of a motion estimation (motion estimation ME). It gives the position of the block used for prediction in the already transmitted reference picture s′(t−1) for each block of size 8×8 and/or 16×16 image points of the actual picture. The result of the motion-compensated prediction is the prediction signal ŝ(t). The residual prediction error e(t)=s(t)−ŝ(t) and the motion vector d(t) are coded at the output of the intraframe encoder IE and transmitted. To obtain the reference picture s′(t−1) the intraframe-encoded prediction error e(t) is again decoded (intraframe decoder ID) and added to the prediction signal ŝ(t). With the help of a picture memory z−1 the reference picture s′(t−1) is prepared. This reference picture s′(t−1) acts both as an input signal for motion compensation MC and also for motion estimation ME. The motion estimation ME supplies a motion vector for the respective block of image points with which the motion compensation MC is controlled, i.e. image points of a block are moved with the aid of the motion vector, with the aid of the actual picture s(t) and the reference picture s′(t−1).
In the current standardized coding method the amplitude resolution of the motion vector amounts to half an image point. For estimation and compensation thus image points must be interpolated in the picture s′(t−1) between the scanning raster, which corresponds to an increase of the scanning rate of about a factor L=2. For example, in the MPEG-4 verification model, as described in the ISO/IEC 14496-2 reference, these image points are produced by bilinear interpolation filtering of the image points in the scanning raster (see FIG. 2). In the following description the picture interpolated from s′(t−1) is designated with s′u(t−1). The interpolated values “+” are produced by interpolation between the scanned values “O” with the interpolation prescription:
A=(A+B)//2, b=(A+B+C+D)//4, c=(A+C)//2. The symbol “//” represents a rounded off whole number division. The interpolation and thus the motion-compensated prediction is disturbed by different aliasing in the picture signal s(t) and the prediction signal ŝ(t) as described in ISO/IEC/SC29/WG11: “Core Experiment on Motion and Aliasing-compensation Prediction (P8)”, Stockholm, July 1997, MPEG97/N1180; in U. Benzler, O. Werner, “Improving Multiresolution Motion Compensating Hybrid Coding by Drift Reduction”, Picture Coding Symposium 1996, March 1996, Melbourne; and in WO 99/04574, so that a greater precision for the motion vector using the simple bilinear interpolation permits no additional improvements of coding efficiency. An improved method for making the prediction signal was suggested in ISO/IEC/SC29/WG11: “Core Experiment on Motion and Aliasing-compensation Prediction (P8)”, Stockholm, July 1997, MPEG97/N1180 and WO 99/04574 for these reasons. The aliasing signal is reduced in the prediction signal by an N-stage aliasing-reducing “Finite Impulse Response”.